The present invention relates to metal surface treatment agents and methods of treating metal surfaces by their utilization. Specifically, it relates to metal surface treatment agents which can form a cured coating to provide anticorrosion coating on metal surfaces, methods of treating metal surfaces by providing anticorrosion treatment by their utilization and pre-coated steel substrates treated with the metal surface treatment agents of the present invention. The present invention specifically relates to methods of treating metal surfaces by providing anticorrosion treatment to the surface of iron, zinc, aluminum, copper, magnesium and their alloys.
The surface of metals such as iron, etc. easily gets rusted without any treatment, so usually various types of anticorrosion treatments are provided to the surface.
As one type of the above described anticorrosion treatments, there has been widely performed a chromate treatment in which electrical and chemical plating of a metal more base than iron is performed such as zinc plating, alloy zinc plating, aluminum plating, etc. and further the metal surface is treated using a treatment agent having a hexavalent chromium such as dichromic acid, chromic anhydride, etc. as a primary component.
Examples of the above described chromate treatments are a reactive chromate treatment such as yellow chromate treatment, coating type chromate treatment, etc.
The anticorrosive action of the chromate treatment so far is not yet completely understood, but It is assumed that chromic acid, contained in treatment agents, is reduced on metal surfaces, such as steel, to form a coating layer and this coating layer has the effect of a passivated action on the treated metal surfaces.
The above described chromate treatment excels in forming a coating excelling in anticorrosion effect on the surface of zinc plated steels and others which easily get rusty. However, recently because of environmental reasons, there has been an active movement to regulate use of hexavalent chromium compounds in metal surface treatment.
The present invention can provide metal surface treatment agents and metal surface treatment methods which have little impact on the environment because of non-use of hazardous compounds such as hexavalent chromium compounds and which can provide desirable anticorrosion effect to metal surfaces when compared with any of chromate treatment, zinc plating, and blast zinc coating treatment.
The present invention can also provide metal surface treatment agents and metal surface treatment methods, which provide anticorrosion coatings having extremely high adhesion to metal surfaces.
The present invention can provide metal surface treatment agents and metal surface treatment methods which can effectively protect the surfaces of metals such as iron, zinc, aluminum, copper, magnesium, etc., stainless steel, iron-zinc alloy, zinc-tin alloy, zinc-nickel alloy, brass, aluminum die cast, magnesium die cast, zinc die cast, and alloys of the above described metals.
Further, the present invention can provide metal surface treatment agents and metal surface treatment methods which can be favorably applied to surfaces of anticorrosion coatings formed by various methods such as electroplating, hot-dipping, plating without electrolysis, alloy plating, DACROTIZED(copyright) treatment (registered trademark), and blast zinc coating.
Corrosion resistant coating steel substrates, in which surfaces of the metal treated with various platings, such as electroplating, hot-dipping, plating without electrolysis, alloy plating, blast zinc coating treatment, etc., for example plated steel sheet, are treated with the metal surface treatment methods of the present invention, allow continuous electric resistance welding such as spot welding, seam welding. Since they excel in bending adhesion and post-bending corrosion resistance, they can be used as pre-coated steel sheets.
In one aspect, the invention is directed to a metal surface treatment agent intended to solve the above described problems which is characterized by containing, as a primary component, at least one kind of mercapto group-containing compounds selected from the following groups and further, optionally, containing at least one substituent selected from the group of curing agents, curing catalysts and silica:
(1) a mercapto group-containing compound (a) which is obtained by condensation from the esterification reaction of polyhydric alcohol having at least two or more hydroxyl groups per molecule and a mercapto group-containing acid compound having at least one or more mercapto groups per molecule and at least one or more carboxyl groups per molecule and also which has an equivalent weight of the mercapto groups in the range of 1 through 18 millimoles/g;
(2) a mercapto group-containing compound (b) which is obtained by condensation from the esterification reaction of polyhydric alcohol having at least two or more hydroxyl groups per molecule, a mercapto group-containing acid compound having at least one or more mercapto groups and at least one or more carboxyl group per molecule, and a polyacid compound having at least two or more carboxyl groups per molecule, and also which has an equivalent weight of the mercapto groups in the range of 1 through 18 millimoles/g;
(3) a mercapto group-containing compound (c) which is obtained by condensation from the esterification reaction of a hydroxyl group-containing resin having the hydroxyl value of 30 mg-KOH/g or greater and a mercapto group-containing acid compound having at least one or more mercapto groups per molecule and at least one or more carboxyl groups per molecule, and also which has an equivalent weight of the mercapto groups in the range of 1 through 18 millimoles/g; and
(4) a mercapto group-containing compound (d) which contains at least one or more mercapto groups and two or more cross-linking functional groups per molecule, and also which has an equivalent weight of the mercapto groups in the range of 1 to 18 millimoles/g.
In another aspect, the invention is directed to a method of treating metal surfaces characterized by including a coating forming process, in which metal surfaces are coated with the above described metal surface treatment agent, and a heat treatment process in which the metal surface treatment agent coating is heated.
In yet another aspect, the invention is directed to a pre-coated metal substrate characterized by having a cured coating which is formed on a plated surface of the metal substrate, treated with zinc or an alloy containing zinc in its composition, using a metal surface treatment agent described above and using a method of treating metal surfaces described above.
A method of treating metal surfaces of the present invention includes a coating forming process, in which metal surfaces are coated with the above described metal surface treatment agent, and a heat treatment process in which the metal surface treatment agent coating is heated.
Below, the above described coating formation process and heat treatment process are described in detail.
1. Coating Formation Process:
(1.1) Metal Surface Treatment Agents:
Examples of metal surface treatment agents used in the above described coating formation process are: compositions having, as a primary component, at least one kind of mercapto group-containing compounds selected from the following group; and compositions containing at least one kind of the mercapto group-containing compounds selected from the group of the mercapto group-containing compounds, (a), (b), (c) and (d) and at least one kind selected from the group of curing agents, and curing catalysts, and silica:
(1) a mercapto group-containing compound (a) which is obtained by condensation from the esterification reaction of polyhydric alcohol having at least two or more hydroxyl groups per molecule and a mercapto group-containing acid compound having at least one or more mercapto groups per molecule and at least one or more carboxyl groups per molecule, and also which has an equivalent weight of the mercapto groups in the range of 1 to 18 millimoles/g (millimoles per gram);
(2) a mercapto group-containing compound (b) which is obtained by condensation from the esterification reaction of polyhydric alcohol having at least two or more hydroxyl groups per molecule, a mercapto group-containing acid compound having at least one or more mercapto groups per molecule and at least one or more carboxyl groups per molecule, and a polyacid compound having at least two or more carboxyl groups per molecule, and also which has an equivalent weight of the mercapto groups in the range of 1 to 18 millimoles/g;
(3) a mercapto group-containing compound (c) which is obtained by condensation by the esterification reaction of a hydroxyl group-containing resin having the hydroxyl value of 30 mg-KOH/g or greater and a mercapto group-containing acid compound having at least one or more mercapto groups per molecule and at least one or more carboxyl groups per molecule, and also which has an equivalent weight of the mercapto groups in the range of 1 to 18 millimoles/g; and
(4) a mercapto group-containing compound (d) which contains at least one or more mercapto groups per molecule and two or more cross-linking functional groups per molecule, and also which has an equivalent weight of the mercapto groups in the range of 1 to 18 millimoles/g.
In the specification: the above mercapto group-containing compounds (a) may be referred to as mercapto group-containing low molecular weight polyester compounds; the above mercapto group-containing compounds (b) may be referred to as mercapto group-containing high molecular weight polyester compounds; the above mercapto group-containing compounds (c) may be referred to as mercapto group-containing resins, and; the above mercapto group-containing compounds (d) may be referred to as mercapto group and cross-linking functional group-containing compounds.
(1.1.1) Mercapto Group-containing Low Molecular Weight Polyester Compounds:
This mercapto group-containing low molecular weight polyester compound is a compound obtained by condensation from the esterification reaction of polyhydric alcohol having at least two or more hydroxyl groups per molecule and a mercapto group-containing acid compound having at least one or more mercapto groups per molecule and at least one or more carboxyl group per molecule. It is preferable that the equivalent weight of the mercapto groups in the compound is 1 millimole/g or greater. When the equivalent weight of the mercapto groups in the mercapto group-containing low molecular weight polyester compound is within the above described range, not only do the mercapto groups in the molecules bond with the metal surface but also intermolecular cross-linking is formed, so a strong coating, excelling in adhesion to the metal surface, is obtained.
From an economic aspect, it is preferable that the equivalent weight of the mercapto groups in the mercapto group-containing low molecular weight polyester compound is 18 millimoles/g or less.
The equivalent weight of the mercapto groups can be measured using spectral analysis such as FT-IR (Fourier Transform infra Red spectroscopy), NMR (Nuclear Magnetic Resonance spectroscopy), etc., and chromatographic analysis such as GPC (Gel Permeation Chromatography) chromatography, etc. or, in the case where the compounding amount between polyhydric alcohol and a mercapto group-containing acid compound is clear, it may be calculated from the compounding. The equivalent weight of the mercapto groups is represented as millimole number of the mercapto groups (xe2x80x94SH) per 1 g of the mercapto group-containing compound. In the specification, all equivalent weights of the mercapto groups are values obtained by calculation from the compounding of the compounds.
Specific examples of the above described mercapto group-containing low molecular weight polyester compounds are compounds obtained by esterifying the hydroxyl groups of polyhydric alcohol with either the mercapto group-containing acid compounds or their derivatives (mercapto group-containing acid compound derivatives). Hereafter, these may collectively be abbreviated as mercapto group-containing acid compounds.
Examples of the above described mercapto group-containing acid compounds are, for example, mercapto group-containing carboxylic acid compounds represented by the following Formula (1).
HOOCxe2x80x94Rxe2x80x94SHxe2x80x83xe2x80x83(1)
In Formula (1), R represents any of a linear alkyl group, a branched alkyl group and a cyclic alkyl group. It is preferable that R contains carbon atoms in the range of 1 to 20 and especially preferable is the range of 1 to 10. When the mercapto group-containing carboxylic acid compounds in which the above described R exceeds 20 carbon atoms, there are instances where the above described range of the equivalent weight of the mercapto groups in the obtained mercapto group-containing low molecular weight polyester compound becomes smaller, thus a coating having sufficient anticorrosion performance may not be obtained.
Examples of the mercapto group-containing carboxylic acid compounds are mercaptoacetic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, 2-mercapto-2-methylpropionic acid, 3-mercapto-2-methylpropionic acid, 4-mercaptobutyric acid, 3-mercaptobutyric acid, 2-mercaptobutyric acid, 5-mercaptovaleric acid, 4-mercaptovaleric acid, 3-mercapto-valeric acid, 2-mercaptovaleric acid, 4-mercapto-3-methylbutyric acid, 3-mercapto-3-methylbutyric acid, 2-mercapto-3-methylbutyric acid, 5-mercapto-4-methylvaleric acid, 4-mercapto-4-methylvaleric acid, 3-mercapto-4-methylvaleric acid and 2-mercapto-4-methylvaleric acid.
Examples of the mercapto group-containing acid compound derivatives are acid halides of the above described mercapto group-containing carboxylic acid compounds represented by Formula (2) below:
XOCxe2x80x94Rxe2x80x94SHxe2x80x83xe2x80x83(2)
where, in the formula, X represents a halogen atom:
and acid anhydrides of the above described mercapto group-containing carboxylic acid compounds represented by Formula (3) below:
O=(OCxe2x80x94Rxe2x80x94SH)2xe2x80x83xe2x80x83(3)
R in Formulas (2) and (3) is the same as R defined in Formula (1), i.e., any of a linear alkyl group, a branched alkyl group and a cyclic alkyl group and similarly, it is preferable that the carbon atoms are in the range of 1 to 20, and specially preferable is 1 to 10.
Specific examples of the mercapto group-containing acid compound derivatives represented by Formulas (2) and (3) are: acid chlorides such as mercaptoacetic acid chlorides, 2-mercaptopropionic acid chlorides, 3-mercaptopropionic acid chlorides, 2-mercapto-2-methylpropionic acid chlorides, 3-mercapto-2-methylpropionic acid chlorides, 4-mercaptobutyric acid chlorides, 3-mercaptobutyric acid chlorides, 2-mercaptobutyric acid chlorides, 5-mercaptovaleric acid chlorides, 4-mercaptovaleric acid chlorides, 3-mercaptovaleric acid chlorides, 2-mercaptovaleric acid chlorides, 4-mercapto-3-methylbutyric acid chlorides, 3-mercapto-3-methylbutyric acid chlorides, 2-mercapto-3-methylbutyric acid chlorides, 5-mercapto-4-methylvaleric acid chlorides, 4-mercapto-4-methylvaleric acid chlorides, 3-mercapto-4-methylvaleric acid chlorides, 2-mercapto-4-methylvaleric acid chlorides, etc.; and acid anhydrides such as mercaptoacetic acid anhydride, 2-mercaptopropionic acid anhydride, 3-mercaptopropionic acid an hydride, 2-mercapto-2-methylpropionic acid an hydride, 3-mercapto-2-methylpropionic acid anhydride, 4-mercaptobutyric acid an hydride, 3-mercaptobutyric acid an hydride, 2-mercaptobutyric acid an hydride, 5-mercaptovaleric acid anhydride, 4-mercaptovaleric acid anhydride, 3-mercaptovaleric acid anhydride, 2-mercaptovaleric acid anhydride, 4-mercapto-3-methylbutyric acid an hydride, 3-mercapto-3-methylbutyric acid an hydride, 2-mercapto-3-methylbutyric acid an hydride, 5-mercapto-4-methylvaleric acid an hydride, 4-mercapto-4-methylvaleric acid an hydride, 3-mercapto-4-methylvaleric acid an hydride, 2-mercapto-4-methylvaleric acid an hydride, etc.
Of these mercapto group-containing acid compounds, the mercapto group-containing carboxylic acid compounds are preferable and especially mercaptoalkylcarboxylic acid compounds containing 1 to 10 carbon atoms is preferable.
Examples of polyhydric alcohols are: dihydric alcohols such as ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol; trihydric alcohols such as glycerin, trimethylolpropane; tetrahydric alcohols such as pentaerythritol, arabinose, xylulose; pentahydric alcohols such as xylose, mannose, glucose, fructose; hexahydric alcohols such as dipentaerythritol; and disaccharides such as maltose, trehalose, and sucrose.
The above described mercapto group-containing low molecular weight polyester compounds can be synthesized, for example, as follows.
As a first synthesis method, there is a method in which the mercapto group-containing acid compound and polyhydric alcohol are heated to cause an esterification reaction, depending upon need, in the presence of an esterification catalyst such as sulfuric acid, hydrochloric acid, p-toluenesulfonic acid, a chloride of zinc and tin, calcium acetate and barium acetate.
The above esterification reaction can be carried out by mixing and heating the mercapto group-containing acid compound and polyhydric alcohol. Further, the esterification reaction can be carried out in the presence of a solvent such as benzene, toluene, nitrobenzene or anisole.
As a second synthesis method, there is a method in which the mercapto group-containing acid compound derivative and polyhydric alcohol are heated to cause a reaction in the presence of a catalyst such as pyridine, etc.
Examples of the mercapto group-containing low molecular weight polyester compounds are, for example, compounds represented by the following Formula (4): 
where, in the formula, R represents the same group as defined in Formula (1).
(1.1.2) Mercapto Group-containing High Molecular Weight Polyester Compounds
The mercapto group-containing high molecular weight polyester compounds are compounds obtained by condensation from the esterification reaction of polyhydric alcohol having at least two or more hydroxyl groups per molecule, a mercapto group-containing acid compound having both at least one or more mercapto groups per molecule and at least one or more carboxyl groups per molecule, and a polyacid compound having at least two or more carboxyl groups per molecule.
It is preferable that the equivalent weight of the mercapto groups in the above described compounds is 1 millimole/g or more. If the equivalent weight of the mercapto groups in the mercapto group-containing high molecular weight polyester compounds is within the range described above, not only do the mercapto groups contained in the molecules bond to the metal surface but also intermolecular cross-linking is formed, so a strong coating excelling in adhesion to the metal surface is obtained.
From an economic aspect, It is preferable that the equivalent weight of the mercapto groups in the mercapto group-containing high molecular weight polyester compounds is 18 millimoles/g or less.
Specific examples of the above mercapto group-containing high molecular weight polyester compounds are compounds obtained by esterifying a part of the hydroxyl groups of polyhydric alcohol with the mercapto group-containing acid compounds, examples of which are shown in Formula (1), Formula (2) and Formula (3) in the above described mercapto group-containing low molecular weight polyester compounds, and also by esterifying the remaining hydroxyl groups with a polyacid compound shown below.
In the mercapto group-containing acid compounds represented by Formulas (1), (2) and (3), the same as above, R represents any of a straight-chained alkyl group, a branched-chain alkyl group and a cyclic alkyl group. It is preferable that R contains carbon atoms in the range of 1 to 20 and especially preferable is the range of 1 to 10. When there is used a mercapto group-containing acid compound in which carbon atoms in R exceed 20, there are instances where the equivalent weight of the mercapto groups in the obtained mercapto group-containing high molecular weight polyester compound becomes less than the above described range, thus a coating having sufficient anticorrosion performance may not be obtained.
As specific examples of the above described mercapto group-containing acid compounds, the compounds described previously can be suitably used.
As specific examples of the polyhydric alcohols, the compounds described previously can be suitably used.
Examples of the polyacid compounds are: saturated dibasic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, 3,6-endmethylenetetrahydrophthalic acid, etc.; unsaturated dibasic acids such as maleic acid, fumaric acid, methaconic acid, citraconic acid, itaconic acid, etc.; and aromatic dibasic acids such as phthalic acid, isophthalic acid, terephthalic acid, tetrachlorophthalic acid, tetrachloroisophthalic acid, and the like.
Examples of the polyacid compound derivatives are dibasic acid anhydrides such as maleic anhydride, succinic anhydride, citraconic anhydride, itaconic anhydride, phthalic anhydride, etc.
The above described mercapto group-containing high molecular weight polyester compounds can by synthesized, for example, as follows:
As a first synthesis method, there is a method in which the mercapto group-containing acid compound, polyhydric alcohol and polyacid compound are heated to cause an esterification reaction and, depending upon need, in the presence of an esterification catalyst such as sulfuric acid, hydrochloric acid, p-toluenesulfonic acid, a chloride of zinc and tin, calcium acetate, barium acetate, etc.
The above esterification reaction can be carried out by mixing and heating the mercapto group-containing acid compound, polyhydric alcohol and polyacid compound. Further, the esterification reaction can be carried out in the presence of a solvent such as benzene, toluene, nitrobenzene, anisole, etc.
As a second synthesis method, there is a method in which the mercapto group-containing acid compound derivative, polyhydric alcohol and polyacid compound are heated to cause a reaction in the presence of a catalyst such as pyridine, etc.
Examples of the mercapto group-containing high molecular weight polyester compounds are compounds represented by the following Formula (5): 
where in the formula, R represents the same group as R defined in Formula (1), Rxe2x80x2 represents an alkyl group which forms a polyester compound and n represents an integer of 1 or greater.
The number of the repeat units (n) in the mercapto group-containing high molecular weight polyester compound can be determined by the blending ratio of the mercapto group-containing acid compound and polyacid compound so that n becomes an integer of 1 or greater. However, it is preferable to prescribe blending so that the weight average molecular weight of the compound is 100,000 or less. When the molecular weight exceeds 100,000, there may be concern of occurrence of problems such as solubility into a diluting solvent and poor appearance when cured.
(1.1.3) Mercapto Group-containing Resins:
Examples of the mercapto group-containing resins are compounds obtained from condensation from the esterification reaction of a known hydroxyl group-containing resin, in which the hydroxyl value is 30 mg-KOH/g or greater as previously described, and a mercapto group-containing acid compound having at least one or more mercapto groups per molecule and at least one or more carboxyl groups per molecule.
It is preferable that the equivalent weight of the mercapto groups in the mercapto group-containing resins is 1 millimole/g or greater, the same as the above described mercapto group-containing low molecular weight polyester compounds and the mercapto group-containing high molecular weight polyester compounds. From the economic aspect, it is also preferable that the equivalent weight of the mercapto groups in the mercapto group-containing resins is 18 millimoles/g or less.
Examples of the mercapto group-containing resins are resins containing the mercapto groups obtained by esterifying at least a part of the hydroxyl groups in a known hydroxyl group-containing resin containing the hydroxyl groups in the molecules with, for example, the mercapto group containing acid compound which is described in the explanation of the mercapto group-containing low molecular weight polyester compounds.
There are no specific limits on hydroxyl group-containing resins used for the synthesis of the above described mercapto group-containing resins, but especially preferable specific examples are alkyd resins having an oil length of 50% or less, oil free alkyd resins, acryl modified alkyd resins, acrylic resins, butyral resins, etc.
The hydroxyl group-containing resins may contain other cross-linking functional groups besides the hydroxyl groups in the molecules. Specific examples of the cross-linking functional groups are a carboxyl group, ester group, vinyl group, carbamoyl group, amino group, imino group, imide group, epoxy group and isocyanate group. The mercapto groups can construct a cross-linking structure by self-crosslinking with heat or by use of a cross-linking agent. However, the introduction of these cross-linking functional groups can produce a composition which permits construction of a cross-linking structure other than that.
As an example of the mercapto group-containing resins, below, in Formula (6), an example of the mercapto group-containing resin which is synthesizable using a polyol acrylic resin having 2-hydroxyethyl methacrylate (2-HEMA) as the primary component repeat units is described. 
where R is the same as the R defined in Formula (1). Et and n respectively represent an ethylene group and the degree of polymerization.
As a reaction method to esterify the hydroxyl groups of the hydroxyl group-containing resins with the above described mercapto group-containing acid compound, there can be used the methods described in the mercapto group-containing low molecular weight polyester compounds.
It is preferable that the hydroxyl group-containing resins have a weight average molecular weight in the range of 1,000 to 100,000 and a hydroxyl value of 30 to 1000 mg-KOH/g. When the hydroxyl group-containing resins having a hydroxyl value of less than 30 mg-KOH/g are used, mercapto group-containing resins having the equivalent weight of the mercapto groups in the above described range may not be obtained. Resins having a hydroxyl value exceeding 1000 mg-KOH/g are practically difficult to prepare. Further, when the hydroxyl group-containing resins with a weight average molecular weight of less than 1,000 are used, a coating having superior resistance may not be formed on metal surfaces. When the hydroxyl group-containing resins with a weight average molecular weight exceeding 100,000 are used, it may be difficult to form a uniform coating on metal surfaces.
In addition, examples of the mercapto group-containing resins are compositions in which various types of thermoplastic resins and thermosetting resins used for coating materials are further compounded with the mercapto group-containing resins obtained by esterifying the hydroxyl groups of the hydroxyl group-containing resins with the mercapto group-containing acid compounds. Examples of the above described thermoplastic resins and thermosetting resins are various types of alkyd resins, acryl modified alkyd resins, acrylic resins, epoxy resins, butyral resins, polyolefin resins, fluorine resins, modified fluorine resins, etc.
(1.1.4) Mercapto Group and Cross-linking Functional Group-containing Compounds:
Such mercapto group-containing compounds are compounds having both one or more mercapto groups and two or more cross-linking functional groups per molecule. In the compounds, it is also preferable that the equivalent weight of the mercapto groups is 1 millimole/g or greater. If the equivalent weight of the mercapto groups in the mercapto group and cross-linking functional group-containing compounds is within the range described above, the mercapto groups contained in the molecules bond to the metal surfaces due to heat and intermolecular crosslinking is formed between the remaining mercapto groups and/or cross-linking functional groups, so a strong coating excelling in adhesion to the metal surface is obtained.
Furthermore, from the economic aspect, it is preferable that the equivalent weight of the mercapto groups in the mercapto group and crosslinking functional group- containing compounds is 18 millimoles/g or less.
As the mercapto group and crosslinking functional group-containing compounds, specifically triazinethiol derivatives having the structure represented below by the following Formula (7) can be favorably used: 
where, RX, Ry and RZ represent xe2x80x94H, xe2x80x94Na, xe2x80x94K, or xe2x80x94NR1R2 and R1 and R2 represent a hydrogen atom, or a linear or branched alkyl group having 20 carbon atoms or less.
In summary, as the above described triazinethiol derivatives, 2,4,6-trithiol-s-triazine and its alkali neutralized salts can be favorably used. Further, in the compounds, it is preferable that the carbon atoms of the alkyl groups represented by R1 and R2 are 20 or less. When they exceed 20, the equivalent weight of the mercapto groups in the compounds becomes less than 1 millimole/g, so there is a possibility that satisfactory coating performance may not be obtained.
In addition, as the mercapto group and crosslinking functional group-containing compounds, specifically there also can be favorably used compounds having the structure represented by the following Formula (8): 
where, R3 to R6represent a single bond or a linear or branched alkyl group having 20 carbon atoms or less. X and Y represent a crosslinking functional group such as a hydroxyl group, a carboxyl group, a sulfone group, a mercapto group and an amino group.
In the above described compounds, It is also preferable that the carbon atoms of groups represented by R1 to R4 are less than 20. When they exceed 20, the equivalent weight of the mercapto groups in the compounds may becomes less than 1 millimole/g, so there is a possibility that satisfactory coating performance may not be obtained.
As specific examples of the compounds, 1-thioglycerol and thiomaleic acid can be favorably used.
(1.1.5) Curing Agent
As the metal surface treatment agents used at the coating formation process, in addition to the above described mercapto group-containing low molecular weight polyester compounds, mercapto group-containing high molecular weight polyester compounds, mercapto group-containing resins and mercapto group and crosslinking functional group-containing compounds (these may be collectively referred to as mercapto group-containing compounds (A)), there can be included metal surface treatment agents containing curing agents (B) which react with these mercapto group-containing compounds (A) and cause them to be cured.
When the metal surface treatment agents containing the curing agent are cured, there is exhibited a technical effect of progression of finer cross-linking reaction compared to the case where no curing agent is used. This dramatically inhibits penetration of corrosive factors such as water, resulting in specially exhibiting a technical effect of improvement in corrosion resistance.
Examples of the above described curing agents (B) are at least one type of compounds selected from the groups of aminoplast compounds, polyisocyanate compounds and epoxy compounds.
Examples of the above described aminoplast compounds are: condensates of an amino compound, such as urea, melamine, benzoguanamine, and the like, and an aldehyde compound, such as formalin, acetoaldehyde, etc.; and alkyl ether compounds of the above described condensates obtained from the reaction of the above described condensates and monohydric alcohol. More specific examples are: methylolated melamines selected from dimethylolated melamine, trimethylolated melamine, tetramethylolated melamine, pentamethylolated melamine and hexamethylolated melamine; methylolated melamine alkyl ethers such as methyl ether of the above methyloy melamine, ethyl ether, propyl ether, isopropyl ether, butyl ether, isobutyl ether, etc.; urea-formamide condensates and urea- melamine condensates.
Examples of the above described polyisocyanate compounds are: organic diisocyanates such as aliphatic diisocyanates, alicyclic diisocyanates, aromatic diisocyanates, etc.; organic diisocyanate polymers in which the above described organic diisocyanates are polymerized; organic diisocyanate addition products which are addition products of the above described organic diisocyanates and compounds selected from polyal, low molecular weight polyester resins, water, etc., and; biuret compounds of the above described organic diisocyanates.
Furthermore, examples of the above described polyisocyanate compounds are block isocyanates obtained from the reaction of any of the above described organic diisocyanates, organic diisocyanate polymers, organic diisocyanate addition products and biuret compounds, and a block agent which is a compound containing active hydrogen. Examples of the compounds used as the block agents are: alcohols such as methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, t-butyl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, propylene glycol monomethyl ether and benzyl alcohol; phenols such as phenol, cresol and xylenol; lactams such as xcex5-caprolactam, xcex4-valerolactam, xcex3-butyrolactam and xcex2-propiolactam; oximes such as formamidexime, acetoaldexime, acetoxime, methyl ethyl ketoxime, diacetylmonooxime, benzophenoneoxime and cyclohexaneoxime; and active methylene compounds such as dimethyl malonate, diethyl malonate, ethyl acetoacetate, methyl acetoacetate and acetylacetone.
Examples of the above described epoxy compounds are bisphenol A epoxy resins, brominated bisphenol A epoxy resins, novolak type epoxy resins, phenol novolak type epoxy resins, alicyclic epoxy resins, polyglycidylamine type phenol resins, alcohol type epoxy resins, ester type epoxy resins and aliphatic epoxy resins.
A preferable ratio between the mercapto group-containing compound w and the curing agent (B) in the above described metal surface treatment agent is that the curing agent (B) is in the range of 1 to 300 parts in terms of solid weight in relation to 100 parts of the mercapto group-containing compound (A). It is especially preferable that the curing agent (B) be in the range of 20xcx9c200 parts, i.e., 20 to 200 parts.
In the above described metal surface treatment agent, compounding the curing agent (B) in the above described ratio causes more complete crosslinking of the mercapto group-containing compound (A) during the heat treatment process described later. Thus, a tougher coating excelling in anticorrosion effect can be formed.
(1.1.6) Additives and Catalysts
Depending upon need, pigments can be added to the above described metal surface treatment agents.
Examples of the pigments are: inorganic coloring pigments such as titanium dioxide, carbon black, iron oxide, etc.; organic coloring pigments such as phthalocyanine blue, phthalocyanine green and Quinacridone red; extender pigments such as barium sulfate, calcium carbonate and silica; and metal powders such as aluminum powder. In addition, pigments that are ordinarily used in the paint industry and ink industry can be used.
Of these, addition of silica as an additive into the metal surface treatment agents can produce technical effects of further improvement in run inhibition, further improvement in corrosion resistance and also improvement in finishing coating adhesion. Examples of silica are synthesized silica, natural silica, etc. and additionally fumed silica, colloidal silica, etc. The finer the silica particle size, the better, and considering compounding convenience and the characteristics of the cured coatings, a preferable particle size is 0.5 to 300 nm (nanometers).
The content of such silica contained in the metal surface treatment agent can be suitably determined in the range so long as the object of the present invention is not compromised, but usually in terms of the solid content, it is 0.05 to 10% by weight and especially 0.1 to 8% by weight.
A curing catalyst, which accelerates a curing reaction, can be added to the above described metal surface treatment agents. The use of the metal surface treatment agents containing a curing catalyst produces technical effects, which allow curing in a shorter time or at a lower temperature condition.
As such curing catalysts, strong acids such as p-toluenesulfonic acid, dinonylnaphthalenedisulfonic acid, dodecylbenzenesulfonic acid, phenylphosphonic acid, monobutyl maleate, butyl phosphate and hydroxyphosphates can be used as a catalyst.
For the above described metal surface treatment agents, organic and/or inorganic oxidizing agents can be favorably used as a curing catalyst.
As the inorganic oxidizing agents, lead dioxide, manganese dioxide, calcium peroxide, zinc peroxide and sodium perborate can be favorably used. As the organic oxidizing agents, favorably used are: fatty acid metal salts like driers; tertiary decanoic acids such as octyl acid, naphthenic acid and neodecanoic acid; and metal salts such as calcium, vanadium, manganese, cobalt, zinc, zirconium, lead and rare earth salts. These can be used in the above described mercapto group-containing compounds (A) without the curing agent (B) or in combination with the curing agent (B).
A content of the above described curing catalysts contained in the metal surface treatment agents is an amount which allows achievement of the object of the present invention and also can be appropriately determined depending upon the type of curing catalysts. Of the curing catalysts, organic oxidizing agents are preferable. Usually the content of the organic oxidizing agent in the metal surface treatment agent is 0.01 to 2% by weight in terms of the solid content and especially 0.01 to 1.5% by weight.
To the above described metal surface treatment agents, furthermore, there can be added additives such as ultraviolet absorbents, light stabilizers, oxidization inhibitors, fillers, suspending agents, run inhibitors, antistatic agents, surface active agents, wetting agents, dispersing agents, etc.
The above described metal surface treatment agents can be prepared as a solution, suspension solution, sol, oil-in-water type emulsion, water-in-oil type emulsion, is aqueous dispersion, non-aqueous dispersion and powder, etc.
When the metal surface treatment agents are prepared as a solution, suspension solution or sol, the following various solvents can be used.
Examples of such solvents are: ketones such as acetone, methyl ethyl ketone, 2-pentanone, 3-pentanone, 2-hexanone, methyl isobutyl ketone, 2-heptanone, 4-heptanone, diisobutyl ketone, etc.; ester type solvents such as ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate, amyl acetate, isoamyl acetate, 3-methoxybutyl acetate, 2-ethylbutyl acetate, 2-ethylhexyl acetate, cyclohexyl acetate, methyl propionate, ethyl propionate, isoamyl propionate, butyrates, isobutyrates, benzoates, ethylene glycol monoesters, ethylene glycol diesters, 2-methoxyethyl acetate, 2-ethoxyethyl acetate, 2-buthoxyethyl acetate, etc.; non-protic amide type solvents such as N,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetoamide, N,N-diethylacetoamide, etc.; non-protic sulfoxide type solvents such as dimethyl sulfoxide, etc.; polar solvents such as non-protic amine type solvents like ternary amine, etc.; and non-polar solvents such as toluene, xylene, cyclohexanone, etc.
In the case of oil-in-water emulsion metal surface treatment agents, the above described metal surface treatment agents can be prepared, for example, by: dissolving the non-water soluble mercapto group-containing compound (A) and the curing agent (B) in a solvent which is not compatible with water or hardly compatible with water selected from the above described ketone type solvents, ester type solvents and mixed solvents; adding to this an appropriate surface active agent to disperse in water; and by further adding pigments, additives, catalysts, etc.
In the case of water-in-oil emulsion metal surface treatment agents, the above described metal surface treatment agents can be prepared, for example, such that: water-soluble mercapto group-containing compound (A) and the curing agent (B) are selected and dissolved in water; and the obtained aqueous solution is dispersed in a hydrocarbon type solvent, such as aliphatic hydrocarbon and aromatic hydrocarbon, etc., by an appropriate surface active agent. Pigments and other additives may be dissolved or suspended in advance in the above described solvent or they may be added to a dispersion solution in which the above described aqueous solution is dispersed in a hydrocarbon type solvent.
In the case of powder metal surface treatment agents, [the above described metal surface treatment agents] can be prepared, for example, by compounding the mercapto group-containing compound (A), the curing agent (B), and, depending upon need, curing catalysts and additives such as silica, etc., with a polyolefin resin, polyvinyl chloride resin or a glycidyl group-modified acrylic resin which are used for powder coating materials, and then pulverizing it.
(1.1) A method of Forming a Coating Using Metal Surface Treatment Agents
(1.2.1) A method of Coating Metal Surfaces
Coating of metal surfaces using the metal surface treatment agents can be performed with various methods depending upon the form of the metal surface treatment agents.
For example, when the metal surface treatment agent is in the form of a liquid such as solution, suspension solution, oil-in-water type emulsion and water-in-oil type emulsion, ordinary coating methods for coating materials sold on the market can be used such as spraying, applicator coating such as bar-coater applicator, brush coating, roll coating, electrostatic coating, dipping, dip-draining, etc. When the metal surface treatment agent is in a powder form, methods such as electrostatic coating, fluidized bed dip coating, etc. can be used.
It is preferable to coat metal surfaces with the metal surface treatment agent so that a coating thickness of 0.01 micron or greater after curing is obtained. When the coating thickness after curing is less than 0.01 micron, sufficient anticorrosion performance may not be obtained. However, from the economic aspect, it is preferable to coat metal surfaces so that the coating thickness after curing is 50 microns or less.
When the metal surface treatment agents are in liquid form, depending upon need, so long as there is no economically adverse effect, it is possible to perform the heat treatment process after setting of the agent.
2. Heat Treatment Process
At the heat treatment process, the coating of the metal surface treatment agents, obtained at the previously described coating forming process, is heat-treated.
Due to the heat treatment described above, at least a part of the mercapto groups in the mercapto group-containing compound (A), contained in the metal surface treatment agents, bonds to the metal surface. And, It is presumed that due to the self-condensation between the remaining mercapto groups, and/or the reaction between the mercapto groups and the curing agent (B), the mercapto group-containing compound (A) is cured to form a tough coating bonded firmly to the metal surface. Heat treatment at the heat treatment process can be performed, for example, in the heating furnace ambient temperature range of 100 to 450xc2x0 C., but 100 to 40xc2x0 C. is preferable. At the coating formation process, when the metal surface treatment agent compounded with the curing agent (B) is used, heat treatment can be performed in the heating furnace ambient temperature range of 80 to 400xc2x0 C., but it is preferable to perform heat treatment in the temperature range of 80 to 380xc2x0 C. The heating time in heat treatment can be, for example, 1 second to 500 minutes and 1 second to 240 minutes are preferable. The heat treatment parameters can be suitably determined by the relation between the above described treatment temperature and treatment time.
Further, when the object to be treated is a pre-coated steel sheet, It is possible to determine heat treatment parameters using the surface temperature of the object to be treated.
3. Products
There are no specific limitations on types of metals and alloys for which the method of treating metal surfaces of the present invention is applied, but especially the method of treating metal surfaces of the present invention can be preferably used on the surface of metals such as iron, zinc, aluminum, copper, magnesium, etc., stainless steel, iron-zinc alloy, zinc-tin alloy, zinc-nickel alloy, brass, aluminum die cast, magnesium die cast, zinc die cast, etc.
The method of treating metal surfaces of the present invention can be also preferably used on the above described metal surfaces treated with electroplating, hot-dipping, plating without electrolysis, alloy plating, etc. Especially preferable are plated steel sheets in which surfaces of base materials such as iron, etc. are plated with electrogalvanizing, fused zinc, fused alumina, fused zinc-aluminum alloy, zinc-nickel alloy, iron-zinc alloy, zinc-tin alloy, etc.
Further, the method of treating metal surfaces of the present invention can be preferably used for: an anticorrosion film formed by DACROTIZING(copyright) treatment (registered trademark) by coating a metal surface with a composition containing zinc powder and a water soluble chromate and then heat-treating it, as disclosed in Tokko [Publication of Examined Patent Application] No. 60-50228; an anticorrosion coating formed by coating a metal surface with a composition containing zinc powder and a specific water soluble resin and heat-treating it, as disclosed in Tokkai [Publication of Unexamined Patent Application] No. 10-46058; and a surface coating formed by performing blast zinc coating treatment (refer to Tokko No. 59-9312) in which a metal surface is blast-treated using a zinc coated blasting material in which a layer of zinc or zinc alloy is formed around the nuclei of iron or iron alloy.
The metal surface treatment agents of the present invention can be applied to various metal forms such as bolts, nuts and screws. In addition, other examples are various different shapes of metal parts, such as sheets, including H-shaped steel plates and I-shaped steel plates.
Below, the present invention is specifically described with reference to examples. The mercapto group-containing compounds A, curing agents (B) and other additives (C) used in preparation of the metal surface treatment agents used in the following examples and comparative examples are described below.
(A) Mercapto Group-containing Compounds (A):
a) SC-1:
According to the synthesis conditions and process described below, 1 mole of ethylene glycol having 2 moles of hydroxyl groups and 2 moles of 3-mercaptopropionic acid having 1 mole of carboxyl groups were esterified and condensed to synthesize a mercapto group-containing compound (SC-1). The structural formula of the compound obtained is represented by Formula (9).
HSxe2x80x94Etxe2x80x94COOxe2x80x94CH2xe2x80x94CH2xe2x80x94OCOxe2x80x94Etxe2x80x94SHxe2x80x83xe2x80x83(9)
in the formula, Et represents an ethylene group.
(1) in a reaction vessel, the specified quantity of ethylene glycol and 3-mercaptopropionic acid were placed and while stirring, the raw material temperature was increased to 180xc2x0 C. in about 2 hours,
(2) they were reacted while maintaining the raw material temperature at 180 to 190xc2x0 C. for 2 hours (progress of esterification reaction), and
(3) the raw material temperature was increased to 210 to 220xc2x0 C. to completely conclude the reaction and the reaction product water produced by esterification was completely removed out of the system and cooled to obtain a mercapto group-containing compound. The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 238 and 8.40 millimoles/g in terms of values of theoretical calculation from the blending of the raw materials.
b) SC-2:
Using trimethylolpropane and 3-mercaptopropionic acid, a compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (a) SC-1 except that 1 mole of 3-mercaptopropionic acid having 1 mole of a carboxyl groups to 1 mole of trimethylolpropane having 3 moles of hydroxyl group were reacted. The structure of the product obtained is represented by the following Formula (10). The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 222 and 4.50 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et having two bonding arms represents an ethylene group and Et having one bonding arm represents an ethyl group.
C) SC-3:
Using trimethylolpropane and 3-mercaptopropionic acid, a compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (a) SC-1 except that 3 moles of 3-mercapto-propionic acid having 1 mole of a carboxyl groups to 1 mole of trimethylolpropane having 3 moles of hydroxyl groups were reacted. The structure of the product obtained is represented by the following Formula (11). The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 398 and 7.54 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et having two bonding arms represents an ethylene group and Et having one bonding arm represents an ethyl group.
d) SC-4:
Using pentaerythritol and 3-mercaptopropionic acid, a compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (a) SC-1 except that 2 moles of 3-mercaptopropionic acid having 1 mole of carboxyl groups to 1 mole of pentaerythritol having 4 moles of hydroxyl groups were reacted. The structure of the product obtained is represented by the following Formula (12). The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 312 and 6.41 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et represents an ethylene group.
e) SC-5:
Using pentaerythritol and 3-mercaptopropionic acid, a compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (a) SC-1 except that 3 moles of 3-mercaptopropionic acid having 1 mole of carboxyl groups to 1 mole of pentaerythritol having 4 moles of hydroxyl groups were reacted. The structure of the product obtained is represented by the following Formula (13). The molecular weight and the equivalent weight of mercapto groups of the compound obtained at this time were respectively 400 and 7.50 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et represents an ethylene group.
f) SC-6:
Using pentaerythritol and mercaptoacetic acid, a compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group- containing compound in (a) SC-1 except that 4 moles of mercaptoacetic acid having 1 mole of carboxyl groups to 1 mole of pentaerythritol having 4 moles of hydroxyl groups were reacted. The structure of the product obtained is represented by the following Formula (14). The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 432 and 9.26 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
g) SC-7:
Using pentaerythritol and 3-mercaptopropionic acid, a compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (a) SC-1 except that 4 moles of 3-mercaptopropionic acid having 1 mole of carboxyl groups to 1 mole of pentaerythritol having 4 moles of hydroxyl groups were reacted. The structure of the product obtained is represented by the following Formula (15). The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 488 and 8.20 millimoles/g in terms of values of theoretical calculation from the compounding of the raw material. 
in the formula, Et represents an ethylene group.
h) SC-8:
Using pentaerythritol and 3-mercaptopropionic acid anhydride, a compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (a) SC-1 except that 2 moles of mercaptoacetic acid containing carboxyl group anhydride (2 moles equivalent in terms of carboxyl group equivalent weight) to 1 mole of pentaerythritol having 4 moles of hydroxyl groups were reacted. The structure of the product obtained is represented by the following Formula (16). The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 488 and 8.20 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et represents an ethylene group.
i) SC-9:
According to the below described synthesis conditions and process, ethylene glycol having 2 moles of hydroxyl groups, 3-mercaptopropionic acid having 1 mole of carboxyl groups and terephthalic acid having 2 moles of carboxyl groups in the mole ratio of 6:2:5 were esterified and condensed to synthesize a mercapto group-containing compound (SC-9). The structural formula of the compound obtained is represented by Formula (17). The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 120.8 and 1.66 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et represents an ethylene group.
(1) Same as SC-1.
(2) Same as SC-1.
(3) The raw material temperature was increased to 210 to 220xc2x0 C. to completely conclude the reaction and the reaction product water produced by esterification was completely removed out of the system,
(4) the raw material was cooled sufficiently until the liquid temperature became 100xc2x0 C. or less, and
(5) the specific amount of terephthalic acid was placed in the reaction vessel and after that, the esterification reaction was caused by performing the same operation as (2) described above.
j) SC-10:
A compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (i) SC-9 except that trimethylolpropane having 3 moles of hydroxyl groups, 3-mercaptopropionic acid having 1 mole of carboxyl groups and terephthalic acid having 2 moles of carboxyl groups were reacted in the mole ratio of 51:53:50 respectively. The structure of the compound obtained is represented by Formula (18).
The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 18098 and 2.93 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et having two bonding arms represents an ethylene group and Et having a single bonding arm represents an ethyl group.
k) SC-11:
A compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (i) SC-9 except that pentaerythritol having 4 moles of hydroxyl groups, mercaptoacetic acid having 1 mole of carboxyl groups and terephthalic acid having 2 moles of carboxyl groups were reacted in the mole ratio of 51:104:50 respectively. The structure of the compound obtained is represented by Formula (19).
The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 21232 and 4.90 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
I) SC-12:
A compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (1) SC-9 except that pentaerythritol having 4 moles of hydroxyl groups, 3-mercaptopropionic acid having 1 mole of carboxyl groups and terephthalic acid having 2 moles of carboxyl groups were reacted in the mole ratio of 51:104:50 respectively. The structure of the compound obtained is represented by Formula (20).
The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 22688 and 4.58 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et represents an ethylene group.
m) SC-13:
A compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (i) SC-9 except that pentaerythritol having 4 moles of hydroxyl groups, 3-mercaptopropionic acid having 1 mole of carboxyl groups and adipic acid having 2 moles of carboxyl groups were reacted in the mole ratio of 26:54:25 respectively. The structure of the compound obtained is represented by Formula (21).
The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 11038 and 4.89 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et represents an ethylene group.
n) SC-14:
A compound was prepared in the same synthesis conditions and process as the synthesis of the above described (i) SC-9 mercapto group-containing compound except that pentaerythritol having 4 moles of hydroxyl groups, 3-mercaptopropionic acid having 1 mole of carboxyl groups and adipic acid having 2 moles of carboxyl groups were reacted in the mole ratio of 51:104:50 respectively. The structure of the compound obtained is represented by Formula (22).
At this time, the molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 21588 and 4.82 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et represents an ethylene group.
o) SC-15:
A compound was prepared in the same synthesis conditions and process as the synthesis of the above described mercapto group-containing compound in (i) SC-9 except that pentaerythritol having 4 moles of hydroxyl groups, 3-mercaptopropionic acid having 1 mole of carboxyl groups and adipic acid having 2 moles of carboxyl groups were reacted in the mole ratio of 201:404:200 respectively. The structure of the compound obtained is represented by Formula (23).
The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 84888 and 4.76 millimoles/g in terms of values of theoretical calculation from the compounding of the raw materials. 
in the formula, Et represents an ethylene group.
p) SC-16:
According to the synthesis conditions and process described below, an acrylic resin containing hydroxyl groups, [Hitalold D1002 (brand name) (weight average molecular weight: 15,000, hydroxyl value: 200 mg-KOH/g) made by hatachi chemical Co. Ltd], for coatings sold on market, and 3-mercaptopropionic acid equivalent to the hydroxyl groups contained in the acrylic resin were esterified and condensed to synthesize a mercapto group-containing compound (SC-16).
Synthesis conditions and process
(1) in a reaction vessel, the specified quantity of acrylic resin containing hydroxyl groups, 3-mercaptopropionic acid and p-toluenesulfonic acid as a catalyst were placed and while stirring, the raw material temperature was increased to 140xc2x0 C. in about 2 hours,
(2) they were reacted while maintaining the raw material temperature at 140 to 150xc2x0 C. for 2 hours (progress of esterification reaction), and
(3) the raw material temperature was increased to 160 to 170xc2x0 C. to completely conclude the reaction and the reaction product water produced by esterification was completely removed out of the system.
The equivalent weight of the mercapto groups of the compound obtained at this time was 2.72 millmoles/g as values of theoretical calculation from the compounding of the raw materials and the hydroxyl value.
q) SC-17
As another example of a mercapto group-containing compound, a compound represented by the following Formula (24), 2,4,6-trithiol-s-triazine, was used as a mercapto group-containing compound (c).
The molecular weight and the equivalent weight of the mercapto groups of the compound were respectively 177 and 16.9 millimoles/g as calculated values. 
r) SC-18
A compound represented by the following Formula (25), 1-thioglycerol, was used as a mercapto group-containing compound (c).
The molecular weight and the equivalent weight of the mercapto groups of the compound were respectively 108 and 9.26 millimoles/g as calculated values. 
S) SC-19
A compound represented by the following Formula (26), thiomaleic acid, was used as a mercapto group-containing compound (c).
The molecular weight and the equivalent weight of the mercapto groups of the compound were respectively 150 and 6.67 millimoles/g as calculated values. 
t) NC-1
As an example of a component not containing mercapto groups (A), pentaerythritol and propionic acid were esterified and condensed to prepare a compound. The synthesis conditions and process were the same as the above described SC-7.
In the esterification reaction, 4 moles of propionic acid having 1 mole of carboxyl groups to 1 mole of pentaervthritol having 4 moles of hydroxyl groups were reacted to prepare the compound. The reaction formula and the structure of the product obtained is represented by the following Formula (27). 
in the formula, Et represents an ethyl group.
u) NC-2
As an example of a compound which has less than 1 millimole/g of the equivalent weight of mercapto groups, a part of the 3-mercaptopropionic acid, which was a raw material used for the synthesis of the mercapto group-containing compound described in the above (m) SC-13, was replaced with propionic acid to prepare a compound. Specifically, a compound was prepared in the same synthesis conditions and process as the synthesis of the mercapto group-containing compound in the above (m) SC-13 except that pentaerythritol having 4 moles of hydroxyl groups, 3-mercaptopropionic acid having 1 mole of carboxyl groups, propionic acid having 1 mole of carboxyl groups and adipic acid having 2 moles of carboxyl groups were reacted in the mole ratio of 26:8:46:25 respectively. The structure of the compound obtained is represented by Formula (28).
The molecular weight and the equivalent weight of the mercapto groups of the compound obtained at this time were respectively 9566 and 0.84 millimoles/g as theoretical calculation values from the compounding of the raw materials. 
in the formula, A represents an ethyl group or a xe2x80x94C2H4xe2x80x94SH group and the ratio is 4:23 in terms of the mole ratio.
(B) curing Agents:
The following two types were used as the curing agent (component B).
a) Melamine Type Curing Agents:
Melamine resin (xe2x80x9cMelamine 1xe2x80x9d)(Melane 2000, solids concentration: 50% by weight), made by Hitachi Chemical, or melamine resin (xe2x80x9cMelamine 2xe2x80x9d)(Sumimal M-50W, solids concentration: 80% by weight), made by Sumitomo Chemical. Benzoguanamine resin (Mycoat 106, solids concentration: 77% by weight), made by Mitsui Cytec, Ltd,.
b) Isocyanate Type Curing Agents:
Block isocyanate (Coronate 2507, solids concentration: 80% by weight), made by Nippon Polyurethane Industries Co., Ltd,.
(C) Other Components:
As other components, the following components (component C) were used.
a) Run Inhibitor:
Fumed silica (Aerosil 200), made by Nippon Aerosil Co., Ltd,.
b) Curing Catalyst:
As a cobalt-containing curing catalyst, there was used a reagent cobalt napthenate (cobalt metal concentration: 6% by weight) made by Wako Pure Chemical Industries, Ltd,.
c) Solvents:
Methyl isobutyl ketone (MIBK), triethyleamine or demineralized water.